1. Field
The present invention relates to a light modulation device. The present invention more particularly relates to a light modulation device for modulating an optical signal.
2. Description of the Related Art
Recently, higher-capacity and longer-distance in an optical transmission system is being promoted. For a method effective in promoting the higher-capacity and longer-distance transmission, introduction of a system using the Multiple Phase Shift Keying represented by DQPSK (Differential Quadrature Phase Shift Keying) is taken notice of.
FIG. 23 shows a general multilevel optical phase modulator. The multilevel optical phase modulator 50 comprises an LD (Laser Diode) 51, a branching section 52, phase modulators 53a and 53b, a 2×1 optical multiplexer 54, a π/2 phase shifter 55, a PD (Photo Diode) 56 and a phase shift controller 57. The modulator 50 is a phase modulator for generating a DQPSK signal.
The LD 51 emits continuous wave light. The branching section 52 branches the continuous wave light into two so as to input one branched light into an optical waveguide of the phase modulator 53a and to input the other branched light into the π/2 phase shifter 55. The π/2 phase shifter 55 shifts the phase of the electric field of the light by π/2 so as to input the phase-shifted light into an optical waveguide of the phase modulator 53b. 
Both of the phase modulators 53a and 53b include a Mach-Zehnder Interferometer. Near parallel waveguides 53a-1 and 53a-2 of the phase modulator 53a, signal electrodes 5a-1 and 5a-2 are provided, respectively. Near parallel waveguides 53b-1 and 53b-2 of the phase modulator 53b, signal electrodes 5b-1 and 5b-2 are provided, respectively. To the signal electrodes 5a-1 and 5a-2, i1 and iln (iln is an inversion signal of i1) as I signals having a complementary relation are inputted. To the signal electrodes 5b-1 and 5b-2, q1 and q1n (q1n is an inversion signal of q1) as Q signals having a complementary relation are inputted.
The phase modulator 53a changes the phase of the input light in a manner corresponding to the 0 s and 1 s of the I signal. The phase modulator 53b changes the phase of the π/2 phase-shifted input light in a manner corresponding to the 0 s and 1 s of the Q signal. The 2×1 optical multiplexer 54 multiplexes modulated light s1 outputted from the phase modulator 53a and modulated light s2 outputted from the phase modulator 53b to generate a DQPSK signal d1.
The PD 56 receives the DQPSK signal d1 and outputs an electric signal according to the light power. The phase shift controller 57 generates a phase control signal based on the electric signal, and adjusts and controls the phase shift amount of the π/2 phase shifter 55.
Here, due to the electric field applied to the optical waveguide of the phase modulator 53a, the refractive indexes of the parallel waveguides 53a-1 and 53a-2 change, respectively. As a result, the phase difference between the parallel waveguides 53a-1 and 53a-2 changes, and thus there is outputted from the output waveguide the modulated light s1 that is intensity-modulated such that the intensity of the optical signal increases if the phase difference between the parallel waveguides 53a-1 and 53a-2 is
0° and decreases if the phase difference is π.
Similarly, due to the electric field applied to the optical waveguide of the phase modulator 53b, the refractive indexes of the parallel waveguides 53b-1 and 53b-2 change, respectively. As a result, the phase difference between the parallel waveguides 53b-1 and 53b-2 changes. In the phase modulator 53b, however, since the optical signal phase-shifted by π/2 at the upstream side is inputted, there is eventually outputted from the output waveguide the modulated light s2 that is intensity-modulated such that the intensity of the optical signal increases if the phase difference between the parallel waveguides 53b-1 and 53b-2 is π/2 and decreases if the phase difference is 3π/2.
FIG. 24 is a phase diagram illustrating Quadrature Phase Shift Keying. The horizontal axis represents a real part Re and the vertical axis represents an imaginary part Im. When the phase modulator 53a modulates light in accordance with the I signal, the modulated signal assumes either 0 (I=0) or π (I=1) on the real axis. When the phase modulator 53b modulates light in accordance with the Q signal, the modulated signal assumes either π/2 (Q=0) or 3π/2 (Q=1) on the imaginary axis, because it is rotated by π/2 with respect to the I signal.
Multiplexing these modulated lights by the 2×1 optical multiplexer 54 means performing additions along the orthogonal real and imaginary axes in the phase diagram, and therefore, the resultant DQPSK signal d1 assumes one of the four phase states π/4 (0,0), 3π/4 (1,0), 5π/4 (1,1) and 7π/4 (0,1) (every adjacent phases are orthogonal).
FIG. 25 shows one example of the phase states of the DQPSK signal d1 outputted from the multilevel optical phase modulator 50. The horizontal axis represents the time and the vertical axis represents the light intensity. From the phase modulator 53a, the modulated light s1 is outputted at a phase such as 0, π, 0, π, 0, 0, 0, . . . . From the phase modulator 53b, the modulated light s2 is outputted at a phase such as π/2, π/2, 3π/2, 3π/2, π/2, π/2, 3π/2, . . . .
In the modulated light s1, only the phases are different from each other such as 0 and π. The signal intensity when the phase is 0 and the signal intensity when the phase is π are the same (in FIG. 24, a distance from the origin of coordinates to 0 on the real axis and a distance from the origin of coordinates to π on the real axis are the same.), and the waveform when the phase is 0 and the waveform when the phase is π are the same.
In the modulated light S2, only the phases are different from each other such as π/2 and 3π/2. The signal intensity when the phase is π/2 and the signal intensity when the phase is 3π/2 are the same (in FIG. 24, a distance from the origin of coordinates to π/2 on the imaginary axis and a distance from the origin of coordinates to 3π/2 on the imaginary axis are the same.), and the waveform when the phase is π/2 and the waveform when the phase is 3π/2 are the same.
Multiplexing these modulated lights s1 and s2 by the 2×1 optical multiplexer 54 means performing additions along the orthogonal real and imaginary axes in the phase diagram, and therefore, the resultant DQPSK signal d1 assumes the phase states π/4, 3π/4, 7π/4, 5π/4, π/4, π/4, 7π/4, . . . .
Thus, the multilevel optical phase modulator 50 performs separate phase modulations using the I and Q signals, respectively, and multiplexes the phase-modulated components together with the phase of the electric field of the light shifted by π/2, thereby performing 4-level quadrature phase shift keying.
Further, the multilevel optical phase modulator 50 performs the feedback control of the optical power monitoring result of the DQPSK signal d1. Thereby, the modulator 50 always performs the phase adjustment such that a phase difference between an optical signal applied to the phase modulator 53a and that applied to the phase modulator 53b is equal to π/2.
For the conventional technique for stabilizing a phase difference based on the output light of the phase modulator, Japanese Unexamined Patent Application Publication No. 2007-43638 (paragraph numbers [0019] to [0021], FIG. 4) and Japanese Unexamined Patent Application Publication No. 2007-82094 (paragraph numbers [0016] to [0032], FIG. 4) are disclosed.
For outputting the above-described DQPSK signal d1, a phase difference between the modulated lights s1 and s2 inputted into the 2×1 optical multiplexer 54 must be exactly equal to π/2.
FIG. 26 shows a concept of the deviation of a phase difference. When the phase difference between the modulated lights s1 and s2 is larger than π/2, the imaginary axis tilts in the left direction with respect to the real axis, whereas when the phase difference is smaller than π/2, the imaginary axis tilts in the right direction with respect to the real axis.
Accordingly, when the π/2 phase difference between the modulated lights s1 and s2 is always kept, namely, when the modulated lights are accurately phase-shifted from each other by π/2 via the phase shift controller 57, the correct DQPSK signal d1 can be generated.
Here, assuming that E0 is an amplitude, continuous wave light is represented by formula (1). Further, the modulated light s1 is represented by formula (2), the modulated light s2 is represented by formula (3), and the DQPSK signal d1 is represented by formula (4). The phase-modulated signal components are here omitted.
                              f          ⁢                                          ⁢          1                =                              E            0                    ⁢          exp          ⁢                                          ⁢                      j            ⁡                          (                              ω                ⁢                                                                  ⁢                t                            )                                                          (        1        )                                          s          ⁢                                          ⁢          1                =                              1                          2                                ⁢                      E            0                    ⁢          exp          ⁢                                          ⁢                      j            ⁡                          (                              ω                ⁢                                                                  ⁢                t                            )                                                          (        2        )                                          s          ⁢                                          ⁢          2                =                              1                          2                                ⁢                      E            0                    ⁢          exp          ⁢                                          ⁢                      j            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  t                                +                ϕ                            )                                                          (        3        )            d1=cos(φ/2)E0expj(ωt+φ/2)  (4)
FIG. 27 shows the DQPSK signal d1. The horizontal axis represents the phase, and the vertical axis represents the light intensity. FIG. 27 shows a function curve of the DQPSK signal d1 represented by formula (4) (cos(φ/2)·E0 in formula (4) represents the light intensity). Further, the output power of the DQPSK signal d1 generated when a phase difference between the modulated lights s1 and s2 is equal to φ=π/2 is expressed as pw (that is, cos(π/2/2)·E0, and therefore, pw=(21/2/2)·E0).
In the multilevel optical phase modulator 50, the phase shift controller 57 applies a phase control signal to the π/2 phase shifter 55 to perform feedback control such that the output power of the DQPSK signal d1 is equal to pw.
However, as is apparent from FIG. 27, the output power pw is an intermediate value between the conditions where the light intensity received by the PD 56 is maximized and those where the light intensity is minimized. Therefore, it is difficult to always detect during the operation the intermediate value of the output intensity, which is the optimal point with respect to the DQPSK signal d1, and to perform the phase shift control such that the output intensity is equal to the intermediate value. Accordingly, to stabilize the phase difference of π/2, a complicated structure and control are required as disclosed in the above-described conventional technique (Japanese Unexamined Patent Application Publications Nos. 2007-43638 and 2007-82094).
On the other hand, there is considered a method of previously measuring the maximum intensity and the minimum intensity of the DQPSK signal d1 to calculate the intermediate value, setting the calculated value in the device and performing the phase shift control such that the output intensity of the signal d1 is equal to this intermediate value. In this case, since the maximum intensity and the minimum intensity change due to aged deterioration of the LD 51, a real intermediate value also changes. As a result, a big error occurs between the set intermediate value and the real intermediate value.